1. Field of the Invention
The field of the invention relates to the analysis and the encoding of sequences of electronic pictures and, more particularly, to the sub-sampling of electronic pictures of this type by the selective removal of certain points in the picture or of certain picture frames or complete pictures.
In a specific example, described in detail here below, the method according to the invention can be applied to the sub-sampling of sequences of high definition pictures designed to be transmitted through a limited throughput channel. A preferred application of this type is the transmission of high definition television on MAC channels.
However, the method of the invention can also be used in any system that analyzes a sequence of pictures (robotics, target tracking, searching for spatio-temporal parameters, etc.) or a sequence of sets of data (in medical or meteorological applications, etc.).
For the application and transmission of HDTV on MAC channels, the prevailing standards that must be met consist in the compression of the HDTV picture signal in the form of a standard TV picture signal, either according to European standards or according to American and Japanese standards.
The transmission standards are shown in table 1.
TABLE 1 ______________________________________ European TV and HDTV standards HDTV TV Compression ______________________________________ vertical 1250 lines 625 lines 2:1 definition Total horizontal 1440 points 720 points 2:1 4:1 definition or 960 points 1920 points if HDTV source 1250/1440/50/1:1 (non-interlaced): compression rate 8:1 ______________________________________
The result thereof is that any sampling procedure should be capable of providing a compression rate of either 1:4 or 1:8, depending on whether the HD picture source is interlaced or non-interlaced respectively.
2. Description of the Prior Art
There are already various, known sub-sampling methods providing for a compression rate of this type.
The prior art examples given below correspond to methods for the sub-sampling of non-interlaced pictures. These methods have also been adapted to interlaced pictures.
A first known sub-sampling method is the Japanese "Muse" method called the "non-stable quincunxial line" method. This method consists in doing a sampling operation on four pictures, the picture 1 being sampled at the points 1 and 3 and the picture 3 being sampled at the points 2 and 4 (FIG. 1). The pictures 2 and 4 are reconstructed in the decoder by using a temporal interpolation method.
The advantage of this method is that good samples are obtained, but it requires high quality picture sequences. The result thereof, moreover, is diminishing of the spatial resolution of the transmitted signal.
A second known method, called the "field skip" method, consists in the systematic elimination of every other frame and in the sampling, at the same lines, of each kept frame (FIG. 2) but in quincunxial form. This method results in a loss of half of the vertical definition.
A third prior art method, developed after the Muse system, consists in doing a sampling operation on four frames, in systematically eliminating the frames 3 and 4. Referring to the depiction of FIG. 1, the method consists in sampling the frame 1, at 1 and 3, and the frame 2, at 2 and 4. This method has the drawback of requiring a temporal management of the sampling memory with a buffer memory arrangement, because the samples 3 are sampled, in time, before the samples 2, but must then be transmitted after these samples 2, during the third frame signal.
In short, the prior art methods disadvantage either spatial resolution (in the Muse method) or temporal resolution, or again, they reduce vertical resolution by half (as in the field skip method).
Furthermore, there is another known system for video signal compression, such as the one described in the patent document PCT-A-8 705 000 (British Broadcasting Corporation) proposing the use of motion vectors defined on the picture to achieve the compression processing with a view to improving the sub-sampling of moving picture zones. The proposed sampling structure is chiefly of structure of the line quincunx type, distributed on each frame of successive sets of four frames. The sampling mask for the sampled frames, from the second frame of each set onwards, is shifted as a function of information on motion vectors, so as to sample picture points complementary to those sampled in the first frame, to maximize the amount of information transmitted and, hence, the picture definition.
However, the principle of the British invention has the drawback of making the sampling structure take the motion vector module entirely into account. This calls for setting up complex systems requiring high consumption in data processing to handle motion vector modules. The complexity further increases for reconstructions implying the temporary storage of the first frames in the sets of four frames, for picture reconstruction.